Review Request: Change activity by rolling the mouse wheel over the showActivityManager widget.

[Simone Gaiarin]

Summarizing:
1)No redundant configuration interface should be added.
2)Only cycle through active activity is fine, but the dbus interface
should provide a method to get the list of active activity to simplify
the work, or it will be necessary to make a dbus call for every
activity to check if it is active. But what is the sense of active or
unactive activity?
3)The tooltip popup of the widget now says: "Click to show the
activity manager". We can add a description like: "Scroll to switch
activity."
With this two changes who do not use activities can't do any mista
since the widget does not do anything even if he scroll over it.
Instead the activity aware user can know how the widget work from the
tooltip.
4)Later, as a final improvement an osd with the activity name can be added.
What do you think?